Page 51 - Occupational Health & Safety, October 2019

The answers to these questions will shape the system design.
Consider these recommendations while selecting arc flash miti- gation techniques: reduce AFIE (Arc Fault Incident Energy) level, or PPE, as much as possible, improve service continuity, reduce exposure to live parts, simplify commissioning and usage to reduce human risk factor, optimize cost for Capex and Opex and save space with the minimum footprint and less construction.
Arc Flash Assessments
Within electrical equipment, the arc flash risk can vary dramati- cally, and this typically results in a maximum “zone” of risk associ- ated with the line-side of the main circuit breaker. Another zone of risk, typically an order of magnitude less, exists on the load side of the main circuit breaker. The question is, what is the sufficient iso- lation in between line side and load side of an electrical enclosure?
A risk assessment should include evaluating the incident energy in each of these zones and creating an understanding of proximity risk, that is, what is the chance that an arc, created downstream of the main circuit breaker, could transfer to the line-side of the main circuit breaker? Serious consequences would result if this were to happen and the worker was wearing PPE appropriate only for the load side risk or vice versa.
The case study below indicates the risk environment.
Figure 2. Typical system with risk environment
How do you know if the zones are well separated and distinct? Unfortunately, given today’s practices, there is no present consen- sus in the industry on how this should be determined. Therefore, whoever performs the arc flash hazard analysis study must rely on “engineering judgment” and is responsible for determining the chance that an arc flash could transfer and when it could not. This can be difficult for a person who is not familiar with arc flash anal- ysis methods or lacks detailed knowledge of electrical equipment construction. At best, this type of uncertainty does not seem well aligned with the potential risk, a situation for which further de- velopment in standards and practices is both warranted and likely.
Furthermore, even if the switchboard is put into an electrically- safe working condition, the act of testing to verify if equipment is de-energized could expose a worker to electrical hazards, so a com- plete risk assessment must also include system operational modes including removal from operation and restoration to operation. These also present gaps in the current practice also subject to “en- gineering judgement.”
One solution to address these gaps is to build a vault to con- tain the arc that provides the “separate and distinct” isolation of the zones, along with improved arc flash management of Zone 1, the line side of the main circuit breaker. This significantly reduces the level of risk the worker faces throughout the equipment.
Closing the Judgement Gap with
Passive Line-Side Arc Isolation
There is a new type of line side arc isolation module that addresses these concerns directly and is likely to be recognized as an incident energy reduction method in the 2021 Edition of NFPA 70E. This new design can potentially be applied in many types of low-volt- age distribution equipment, providing positive isolation between protective zones and reducing the chance that employees are in- adequately protected based on inaccurate or incomplete arc flash risk assessments.
The system addresses arcing faults in the equipment at the loca- tion where incident energy levels are typically the highest, the line side of the main circuit breaker. The main circuit breaker can also be used as a lockout/tagout point for all work within the equipment outside of the main breaker. Features such as embedded thermal sensors and “absence of voltage” detectors can further help in day- to-day operation and maintenance of the breaker as well.
Figure 3. shows the same case study shown in Figure 2. with the line side isolation system. This solution reduces the AFIE on the line side and does not allow any plasma or arc transfer from load to line or line to load side. In other words, it is a system to minimize the human risk factor and reduces the AFIE level.
Figure 3. Typical system with the line side isolation module
Conclusion
Electrical hazards pose a significant safety and financial risk for businesses and their employees. OSHA mandates that any work on electrical equipment must be performed in a safe manner that poses no undue risk of injury. To comply with these regulations, businesses must employ the latest protective designs such as line side arc isolation to enhance workplace safety. When businesses use the latest technologies available coupled with internal processes like risk assessment and evaluation, they protect their greatest as- set: their people.
Erhan Cokal, PE, is currently working as an offer marketing leader for low voltage power distribution systems based in Nashville, TN.
REFERENCES
1. NFPA 70E 2018
2. Antony Parsons, Jonathan Gray, Living With Arc-Flash Mitigation, May 2019, IEEE Industry Applications Magazines.
3. Antony Parsons, Tim Faber, Mark A. Metzdorf Enhancing Worker and Equipment Protection Through Passive Arc-Fault Mitigation
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OCTOBER 2019 | Occupational Health & Safety 47